Metal melting furnaces are used to produce refined metal and metal alloys such as steel, stainless steel, nickel, cobalt, aluminum, and so forth. An electric induction furnace is an example of such a furnace. A metal melting furnace has an interior volume for containing the charge to the furnace. The interior volume is initially charged with unmelted scrap. After melting the initial charge, typically, but not necessarily, the interior volume is incompletely filled with molten metal, leaving some free interior volume which is occupied principally with atmospheric air, unless another atmosphere is provided.
Access to the furnace interior volume is desired during the melting period to visually inspect the progress of the melting and to withdraw samples of the melt. Access is also desired to add constituents to the charge as the melting progresses to adjust the melt to the required composition of alloy.
Molten metals react with, dissolve and absorb atmospheric air in varying degrees causing oxidation, slag formation and compositionally unsatisfactory product. The results are poor metal properties, poor casting quality, decreased yields and increased production cost.
To circumvent this problem, cover lids are used to restrict the infiltration of atmospheric air into the interior volume of the furnace. Sometimes an inert gas may also be introduced under the lid to reduce or further restrict infiltration of air. Such cover lids, however, block physical and visual access to the furnace opening and are infrequently used by operators.
Another approach has been to introduce a protective gas through a conduit directly into the free volume of the furnace. However, large volumes of protective gas are required which can be expensive depending on the protective gas used.
Still another approach has been to introduce a liquified protective gas onto the surface of the melt. This approach has the danger of metal explosion if liquid gas becomes trapped below the surface of the melt. Also the oxygen concentrations developed in the free interior furnace volume are undesirably high for the amount of liquified gas used.
Yet another method is to provide a single layer fluid curtain or jet of protective gas across the opening to the furnace. Concurrently a flow of protective gas may be introduced directly into the free furnace volume as a supplementary purge. The use of a turbulent jet or single layer curtain is wasteful of protective gas in comparison to the multi-layer curtain employed in this invention.
The prior art describes the generation of a fluid curtain by issue of fluid from slots, nozzles, and porous surfaces. The present invention provides a novel device for the generation of a fluid curtain which has greater capability of excluding atmospheric air from entering an opening.